1. Field of the Invention
The present invention relates to a multilayer ceramic substrate having internal conductors such as via-hole conductors, for example, and particularly to a technique of preventing the generation of defects around the internal conductors.
2. Description of the Prior Art
While a substrate for mounting an electronic device thereon has been put to extensive use in the fields of electronics etc., multilayer ceramic substrates have been proposed and put to practical use as highly reliable substrates in reply to recent demands for making electronics small-sized, lightweight and multifunctional. The multilayer ceramic substrate comprises a plurality of ceramic layers stacked and has wiring conductors or electronic elements incorporated integrally into each ceramic layer to achieve high-density mounting.
The multilayer ceramic substrate can be obtained through the steps of stacking a plurality of green sheets to form a multilayer body and firing the multilayer body. The green sheet always shrinks as accompanied by sintering in the firing step. This is a serious cause in decreasing the dimensional accuracy of the multilayer ceramic substrate. To be concrete, a shrinkage variation arises as accompanied by the shrinkage and, in the multilayer ceramic substrate being obtained finally, the dimensional accuracy falls around 0.5%.
Under these circumstances, a so-called non-shrinkage firing process capable of suppressing the shrinkage in the in-plane direction and shrinking only in the thickness direction of the green sheets in the firing step of the multilayer ceramic substrate was proposed (refer to JP-A HEI 10-75060, for example). As described in the prior art, when a multilayer body of green sheets having attached thereto a sheet not shrinkable even at the temperature in the firing step is fired, the shrinkage in the in-plane direction is suppressed and only the shrinkage in the thickness direction is produced. According to this process, the dimensional accuracy in the in-plane direction of a multilayer ceramic substrate can be improved to stay at around 0.05%, for example.
Incidentally, the formation of internal conductors for implementing interlayer connection, such as via-hole conductors, is indispensable to a multilayer ceramic substrate. When fabricating the multilayer ceramic substrate, a firing step is taken, with conductive paste filled in via holes formed. In this case, it has been known to the art that pores (defects) are produced around the internal conductors (via-hole conductors, for example) by dint of the difference in thermally shrinking behavior between the conductive paste and the green sheets. The generation of such defects arises copiously, particularly in the non-shrinkage firing process.
Therefore, techniques of eliminating these defects have been studied in various fields (refer to JP-A 2003-133745 and Japanese Patent Nos. 2732171 and 3589239, for example). The invention described in JP-A 2003-133745 uses a conductive composition for a multilayer ceramic substrate containing conductive powder, such as of Ag, and an Mo compound or Mo metal as a conductive composition to be filled in via holes to enable the production of a multilayer ceramic substrate giving rise to no defect in the vicinity of electrodes after being fired. Similarly, in the invention described in Japanese Patent No. 2732171, a via-hole conductor is formed of Ag and W to attempt not to generate any pore between the via-hole conductor and the inner wall of the via hole. The invention described in Japanese Patent No. 3589239 uses as a conductive component of conductive paste conductive powder coated with a metal oxide to elevate the shrinkage-starting temperature of the conductive paste, thereby trying not to generate any stress that possibly restrains shrinkage during the shrinkage of a compacted ceramic body by firing.
According to the repeated studies by the present inventors, however, it has been found that satisfactory results cannot always be obtained only from the control of the shrinking behavior of the conductive paste for forming the internal conductors as disclosed in each of the prior art references and in particular that the generation of the defects around the internal conductors cannot sufficiently be suppressed in the case of fabricating a multilayer ceramic substrate using the non-shrinkage firing process. In addition, in the case of covering the surface of the conductive powder with a metal oxide as disclosed in Japanese Patent No. 3589239, for example, there is a possibility of an increase in electric resistance of the internal conductors becoming conspicuous.
The present invention has been proposed in view of the conventional state of affairs, and the object thereof is to enable infallible elimination of possible generation of defects around the internal conductors of the multilayer ceramic substrate and thereby to provide a highly reliable multilayer ceramic substrate and a production method thereof.
To attain the above object, the present inventors have been made various studies continuously over a long period of time and, as a result, have come to a conclusion that diffusion of Ti, Zr and/or Mn around the internal conductors enables the generation of the defects to be effectively suppressed. The present invention has been perfected based on this knowledge.